The performance of tungsten carbide end-mills in micro-milling of Ti-6Al-4V under nitrogen cooling

Engelbrecht, Maria Elizabeth (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: A strong demand exists from various industries to fabricate miniature devices and components with complex microscale features from a wide range of materials including Ti-6Al-4V. The need exists to improve the micro-machining process of using tungsten carbide end-mills to micro-mill Ti-6Al-4V. Key performance indicators are the rate of tool wear, tool life, surface finish as well as the forces experienced when machining. In this research, the performance of tungsten carbide end-mills were investigated when micro-milling Ti-6Al-4V under nitrogen gas cooling. A rotatable central composite design of experiments was applied to generate the run-order of the experiments. Tungsten carbide end-mills with a diameter of 1.5 mm were used to micro-mill Ti-6Al-4V. Each experimental run consisted of machining 6 slots of 70 mm each. The experimental variables were cutting speed, feed rate and depth of cut. The tungsten carbide micro-tools were analysed before and after cutting to see the effect of the experimental conditions and Ti-6Al-4V workpiece material on the micro-tools. Cutting forces were also measured throughout the whole experimental procedure using an ATI Net F/T Gamma Sensor to measure forces and deduce the effects of the experimental conditions on the forces experienced by the tungsten carbide micro-tools at the tool workpiece interface. After each experimental run, the metal chips were gathered for analysis. The Ti-6Al-4V workpieces underwent microscopy to document the tool wear by measuring the width of the machined slots as well as atomic force microscopy to measure the surface roughness of each experiment. The purpose of the experiments were to find settings for the three control factors that will maximise tool life and simultaneously minimise all the other responses. In order to achieve this, a mathematical model was fitted for each of the responses in terms of the three control factors. Once the models were established, numerical methods were used to find the optimal settings. For spindle speed (15000 – 17000 RPM), feed rate per tooth (20 – 28 m) and depth of cut (93.75 – 156.25 m), the results of the models predicted that an optimal solution can be found. To produce the best tool life, the values of the three factors should be 17000 RPM for spindle speed, 156.25 m for depth of cut and 28 m for feed per tooth. This yielded an overall desirability of 0.780. However, by increasing the limits of the three factors slightly to outside the experimental space the model can predict an even better solution. For ranges of spindle speed (15000 – 20000 RPM), feed rate per tooth (20 – 35 µm) and depth of cut (93.75 – 250 µm) the best tool life of 1800 mm was found at 19000 RPM for spindle speed, 243.10 µm for depth of cut and 34.80 µm for feed per tooth. This yielded a desirability of 1.000. Since the predicted optimal parameters lie outside of the experimental space it is suggested that future studies should explore this region.

AFRIKAANSE OPSOMMING: Daar is ‘n sterk aanvraag uit verskeie industrië om miniatuur toestelle en komponente met komplekse mikroskaal eienskappe te fabriseer uit ‘n wye reeks materiale, insluitende Ti-6Al-4V. Daar bestaan ‘n behoefte om die mikrofrees proses van Ti-6Al-4V, met die gebruik van tungsten karbied mikro beitels te verbeter. Die hoof faktore om te ondersoek is die tempo van beitel slytasie, oppervlak gladheid en die kragte betrokke by die masjineringsproses. Hierdie navorsing fokus op hoe tungsten karbied mikro beitels vaar wanneer hulle Ti-6Al-4V sny terwyl hulle deur koue stikstof gas verkoel word. ‘n Roteerbare sentrale saamgestelde ontwerp van eksperimente was saamgestel om die ekperimentele lopie volgorde te bepaal. Tungsten karbied mikro beitels met ‘n diameter van 1.5 mm was gebruik om Ti-6Al-4V te mikrofrees. Elke eksperimentele lopie het bestaan uit 6 gemasjineerde gleuwe van 70 mm elk. Die eksperimentele veranderlikes was snyspoed, voertempo per tand en sny diepte. Die tungsten karbied mikro beitels is geanaliseer voor en na die snywerk om die effek van die eksperimentele kondisies en die Ti-6Al-4V op die beitels waar te neem. Snykragte is deur die duur van die eksperimente geneem deur gebruik van ‘n ATI Net F/T Gamma Sensor om die effekte van die eksperimentele kondisies by die beitel-werkstuk interaksie area waar te neem. Na elke eksperimentele lopie is die metaal snysels versamel om ook geanaliseer te word. Die Ti-6Al-4V plate is onder ‘n mikroskoop ondersoek om die wydte van die gemasjineerde gleuwe te meet. Die plate is ook deur ‘n atomiese krag mikroskoop ondersoek om die oppervlak gladheid van die gleuwe te meet. Die doel van die eksperimente was om die masjien stelbare stellings vir die drie veranderlikes te kry wat die beitel lewe maksimeer en terselfde tyd al die ander meetbare faktore minimeer. ‘n Wiskundige model is op al die meetbare faktore gepas in terme van die drie veranderlike waardes. Na die model opgestel is, is statistiese metodes gebruik om die optimale stellings te bepaal. Die resultate van die model het aangedui dat daar vir snyspoed ((15000 – 17000 RPM), voertempo per tand (20 – 28 ʯm) en sny diepte (93.75 – 156.25 ʯm) ‘n optimale oplossing voorspel kan word. Om die beste beitel lewe te bereik moet snyspoed gestel word teen 17000 RPM, voertempo per tand moet 28 ʯm wees en ‘n diepte van 156.25 ʯm moet gesny word. Hierdie stellings het ‘n totale begeerlikheid van 0.780. Nietemin, deur die drie faktore se waardes ‘n klein bietjie na buite die eksperimentele area te verstel kan moontlik selfs ‘n beter oplossing opgelewer word. Die resultate van die ander voorspelde model het aangedui dat daar vir snyspoed ((15000 – 20000 RPM), voertempo per tand (20 – 35 ʯm) en sny diepte (93.75 – 250 ʯm) ‘n beter optimale oplossing voorspel kan word. Om die beste beitel lewe van 1800 mm te bereik moet snyspoed gestel word teen 19000 RPM, voertempo per tand moet 34.80 ʯm wees en ‘n diepte van 243.10 ʯm moet gesny word. Hierdie stellings het ‘n totale begeerlikheid van 1.00. Aangesien die voorspelde optimale waardes buite die eksperimentele area is word daar voorgestel dat toekomstige studies hierdie area moet ondersoek.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98656
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